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    GitHub
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class="sidebar-sub-header level2"><a href="/noteslibrary/pages/e85e68947502cf90/#object-is" class="sidebar-link">Object.is()</a></li><li class="sidebar-sub-header level2"><a href="/noteslibrary/pages/e85e68947502cf90/#object-assign" class="sidebar-link">Object.assign()</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/noteslibrary/pages/e85e68947502cf90/#基本用法" class="sidebar-link">基本用法</a></li><li class="sidebar-sub-header level3"><a href="/noteslibrary/pages/e85e68947502cf90/#注意点" class="sidebar-link">注意点</a></li><li class="sidebar-sub-header level3"><a href="/noteslibrary/pages/e85e68947502cf90/#常见用途" class="sidebar-link">常见用途</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/noteslibrary/pages/e85e68947502cf90/#object-getownpropertydescriptors" class="sidebar-link">Object.getOwnPropertyDescriptors()</a></li><li class="sidebar-sub-header level2"><a 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href="/noteslibrary/pages/e85e68947502cf90/#object-keys" class="sidebar-link">Object.keys()</a></li><li class="sidebar-sub-header level3"><a href="/noteslibrary/pages/e85e68947502cf90/#object-values" class="sidebar-link">Object.values()</a></li><li class="sidebar-sub-header level3"><a href="/noteslibrary/pages/e85e68947502cf90/#object-entries" class="sidebar-link">Object.entries()</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/noteslibrary/pages/e85e68947502cf90/#object-fromentries" class="sidebar-link">Object.fromEntries()</a></li></ul></li><li><a href="/noteslibrary/pages/02c86eb2792f3262/" class="sidebar-link">Symbol</a></li><li><a href="/noteslibrary/pages/0c21dae358fca16b/" class="sidebar-link">Set 和 Map 数据结构</a></li><li><a href="/noteslibrary/pages/f56ec2ab97d60483/" class="sidebar-link">Proxy</a></li><li><a href="/noteslibrary/pages/74de3e45e4491e95/" class="sidebar-link">Reflect</a></li><li><a href="/noteslibrary/pages/2810ae8985e9bd52/" class="sidebar-link">Promise 对象</a></li><li><a href="/noteslibrary/pages/48df907ad3570f3d/" class="sidebar-link">Iterator 和 for-of 循环</a></li><li><a href="/noteslibrary/pages/718b48ed9ce0adce/" class="sidebar-link">Generator 函数的语法</a></li><li><a href="/noteslibrary/pages/75af7031eb66847b/" class="sidebar-link">Generator 函数的异步应用</a></li><li><a href="/noteslibrary/pages/3777253e65bac487/" class="sidebar-link">async 函数</a></li><li><a href="/noteslibrary/pages/e831e1593c82bbe0/" class="sidebar-link">Class 的基本语法</a></li><li><a href="/noteslibrary/pages/83f8c3a0cd87dd83/" class="sidebar-link">Class 的继承</a></li><li><a href="/noteslibrary/pages/efe2fb04eb8ac5fb/" class="sidebar-link">Module 的语法</a></li><li><a href="/noteslibrary/pages/a79ca2e64ceae213/" class="sidebar-link">Module 的加载实现</a></li><li><a href="/noteslibrary/pages/984bf549204bb266/" class="sidebar-link">编程风格</a></li><li><a href="/noteslibrary/pages/32c35f7651d6e58e/" class="sidebar-link">读懂 ECMAScript 规格</a></li><li><a href="/noteslibrary/pages/16121351be68691b/" class="sidebar-link">异步遍历器</a></li><li><a href="/noteslibrary/pages/a2ba314746bfdbdd/" class="sidebar-link">ArrayBuffer</a></li><li><a href="/noteslibrary/pages/7188882b8d65af1b/" class="sidebar-link">最新提案</a></li><li><a href="/noteslibrary/pages/e97bc1e5626b082c/" class="sidebar-link">装饰器</a></li><li><a href="/noteslibrary/pages/1cf50330655efc69/" class="sidebar-link">函数式编程</a></li><li><a href="/noteslibrary/pages/6a8e2dc558da1b39/" class="sidebar-link">Mixin</a></li><li><a href="/noteslibrary/pages/8e8f80f69b775a56/" class="sidebar-link">SIMD</a></li><li><a href="/noteslibrary/pages/ea6f3b870f6dab69/" class="sidebar-link">参考链接</a></li></ul></section></li></ul> </aside> <div><main class="page"><div class="theme-vdoing-wrapper "><div class="articleInfo-wrap" data-v-06225672><div class="articleInfo" data-v-06225672><ul class="breadcrumbs" data-v-06225672><li data-v-06225672><a href="/noteslibrary/" title="首页" class="iconfont icon-home router-link-active" data-v-06225672></a></li> <li data-v-06225672><a href="/noteslibrary/fontend/#前端" data-v-06225672>前端</a></li><li data-v-06225672><a href="/noteslibrary/fontend/#ES6笔记" data-v-06225672>ES6笔记</a></li></ul> <div class="info" data-v-06225672><div title="作者" class="author iconfont icon-touxiang" data-v-06225672><a href="javascript:;" data-v-06225672>阮一峰</a></div> <div title="创建时间" class="date iconfont icon-riqi" data-v-06225672><a href="javascript:;" data-v-06225672>2020-02-09</a></div> <!----></div></div></div> <!----> <div class="content-wrapper"><div class="right-menu-wrapper"><div class="right-menu-margin"><div class="right-menu-title">目录</div> <div class="right-menu-content"></div></div></div> <h1><img src="">对象的新增方法<!----></h1> <!----> <div class="theme-vdoing-content content__default"><h1 id="对象的新增方法"><a href="#对象的新增方法" class="header-anchor">#</a> 对象的新增方法</h1> <p>本章介绍 Object 对象的新增方法。</p> <h2 id="object-is"><a href="#object-is" class="header-anchor">#</a> Object.is()</h2> <p>ES5 比较两个值是否相等，只有两个运算符：相等运算符（<code>==</code>）和严格相等运算符（<code>===</code>）。它们都有缺点，前者会自动转换数据类型，后者的<code>NaN</code>不等于自身，以及<code>+0</code>等于<code>-0</code>。JavaScript 缺乏一种运算，在所有环境中，只要两个值是一样的，它们就应该相等。</p> <p>ES6 提出“Same-value equality”（同值相等）算法，用来解决这个问题。<code>Object.is</code>就是部署这个算法的新方法。它用来比较两个值是否严格相等，与严格比较运算符（===）的行为基本一致。
</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function">is</span><span class="token punctuation">(</span><span class="token string">'foo'</span><span class="token punctuation">,</span> <span class="token string">'foo'</span><span class="token punctuation">)</span>
<span class="token comment">// true</span>
Object<span class="token punctuation">.</span><span class="token function">is</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token comment">// false</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>不同之处只有两个：一是<code>+0</code>不等于<code>-0</code>，二是<code>NaN</code>等于自身。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token operator">+</span><span class="token number">0</span> <span class="token operator">===</span> <span class="token operator">-</span><span class="token number">0</span> <span class="token comment">//true</span>
<span class="token number">NaN</span> <span class="token operator">===</span> <span class="token number">NaN</span> <span class="token comment">// false</span>

Object<span class="token punctuation">.</span><span class="token function">is</span><span class="token punctuation">(</span><span class="token operator">+</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token operator">-</span><span class="token number">0</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
Object<span class="token punctuation">.</span><span class="token function">is</span><span class="token punctuation">(</span><span class="token number">NaN</span><span class="token punctuation">,</span> <span class="token number">NaN</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>ES5 可以通过下面的代码，部署<code>Object.is</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>Object<span class="token punctuation">,</span> <span class="token string">'is'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
  <span class="token function-variable function">value</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">x<span class="token punctuation">,</span> y</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>x <span class="token operator">===</span> y<span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token comment">// 针对+0 不等于 -0的情况</span>
      <span class="token keyword">return</span> x <span class="token operator">!==</span> <span class="token number">0</span> <span class="token operator">||</span> <span class="token number">1</span> <span class="token operator">/</span> x <span class="token operator">===</span> <span class="token number">1</span> <span class="token operator">/</span> y<span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token comment">// 针对NaN的情况</span>
    <span class="token keyword">return</span> x <span class="token operator">!==</span> x <span class="token operator">&amp;&amp;</span> y <span class="token operator">!==</span> y<span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">,</span>
  <span class="token literal-property property">configurable</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
  <span class="token literal-property property">enumerable</span><span class="token operator">:</span> <span class="token boolean">false</span><span class="token punctuation">,</span>
  <span class="token literal-property property">writable</span><span class="token operator">:</span> <span class="token boolean">true</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div><h2 id="object-assign"><a href="#object-assign" class="header-anchor">#</a> Object.assign()</h2> <h3 id="基本用法"><a href="#基本用法" class="header-anchor">#</a> 基本用法</h3> <p><code>Object.assign</code>方法用于对象的合并，将源对象（source）的所有可枚举属性，复制到目标对象（target）。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> target <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token number">1</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token keyword">const</span> source1 <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">b</span><span class="token operator">:</span> <span class="token number">2</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> source2 <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">c</span><span class="token operator">:</span> <span class="token number">3</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>

Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> source1<span class="token punctuation">,</span> source2<span class="token punctuation">)</span><span class="token punctuation">;</span>
target <span class="token comment">// {a:1, b:2, c:3}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p><code>Object.assign</code>方法的第一个参数是目标对象，后面的参数都是源对象。</p> <p>注意，如果目标对象与源对象有同名属性，或多个源对象有同名属性，则后面的属性会覆盖前面的属性。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> target <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token literal-property property">b</span><span class="token operator">:</span> <span class="token number">1</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token keyword">const</span> source1 <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">b</span><span class="token operator">:</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token literal-property property">c</span><span class="token operator">:</span> <span class="token number">2</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> source2 <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">c</span><span class="token operator">:</span> <span class="token number">3</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>

Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> source1<span class="token punctuation">,</span> source2<span class="token punctuation">)</span><span class="token punctuation">;</span>
target <span class="token comment">// {a:1, b:2, c:3}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>如果只有一个参数，<code>Object.assign</code>会直接返回该参数。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span> <span class="token operator">===</span> obj <span class="token comment">// true</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>如果该参数不是对象，则会先转成对象，然后返回。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">typeof</span> Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// &quot;object&quot;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>由于<code>undefined</code>和<code>null</code>无法转成对象，所以如果它们作为参数，就会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span><span class="token keyword">undefined</span><span class="token punctuation">)</span> <span class="token comment">// 报错</span>
Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token comment">// 报错</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>如果非对象参数出现在源对象的位置（即非首参数），那么处理规则有所不同。首先，这些参数都会转成对象，如果无法转成对象，就会跳过。这意味着，如果<code>undefined</code>和<code>null</code>不在首参数，就不会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token keyword">undefined</span><span class="token punctuation">)</span> <span class="token operator">===</span> obj <span class="token comment">// true</span>
Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token operator">===</span> obj <span class="token comment">// true</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>其他类型的值（即数值、字符串和布尔值）不在首参数，也不会报错。但是，除了字符串会以数组形式，拷贝入目标对象，其他值都不会产生效果。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> v1 <span class="token operator">=</span> <span class="token string">'abc'</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> v2 <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> v3 <span class="token operator">=</span> <span class="token number">10</span><span class="token punctuation">;</span>

<span class="token keyword">const</span> obj <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> v1<span class="token punctuation">,</span> v2<span class="token punctuation">,</span> v3<span class="token punctuation">)</span><span class="token punctuation">;</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// { &quot;0&quot;: &quot;a&quot;, &quot;1&quot;: &quot;b&quot;, &quot;2&quot;: &quot;c&quot; }</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码中，<code>v1</code>、<code>v2</code>、<code>v3</code>分别是字符串、布尔值和数值，结果只有字符串合入目标对象（以字符数组的形式），数值和布尔值都会被忽略。这是因为只有字符串的包装对象，会产生可枚举属性。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token function">Object</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token comment">// {[[PrimitiveValue]]: true}</span>
<span class="token function">Object</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span>  <span class="token comment">//  {[[PrimitiveValue]]: 10}</span>
<span class="token function">Object</span><span class="token punctuation">(</span><span class="token string">'abc'</span><span class="token punctuation">)</span> <span class="token comment">// {0: &quot;a&quot;, 1: &quot;b&quot;, 2: &quot;c&quot;, length: 3, [[PrimitiveValue]]: &quot;abc&quot;}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码中，布尔值、数值、字符串分别转成对应的包装对象，可以看到它们的原始值都在包装对象的内部属性<code>[[PrimitiveValue]]</code>上面，这个属性是不会被<code>Object.assign</code>拷贝的。只有字符串的包装对象，会产生可枚举的实义属性，那些属性则会被拷贝。</p> <p><code>Object.assign</code>拷贝的属性是有限制的，只拷贝源对象的自身属性（不拷贝继承属性），也不拷贝不可枚举的属性（<code>enumerable: false</code>）。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token literal-property property">b</span><span class="token operator">:</span> <span class="token string">'c'</span><span class="token punctuation">}</span><span class="token punctuation">,</span>
  Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token string">'invisible'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
    <span class="token literal-property property">enumerable</span><span class="token operator">:</span> <span class="token boolean">false</span><span class="token punctuation">,</span>
    <span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token string">'hello'</span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token punctuation">)</span>
<span class="token comment">// { b: 'c' }</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，<code>Object.assign</code>要拷贝的对象只有一个不可枚举属性<code>invisible</code>，这个属性并没有被拷贝进去。</p> <p>属性名为 Symbol 值的属性，也会被<code>Object.assign</code>拷贝。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span><span class="token punctuation">{</span> <span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token string">'b'</span> <span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span> <span class="token punctuation">[</span><span class="token function">Symbol</span><span class="token punctuation">(</span><span class="token string">'c'</span><span class="token punctuation">)</span><span class="token punctuation">]</span><span class="token operator">:</span> <span class="token string">'d'</span> <span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token comment">// { a: 'b', Symbol(c): 'd' }</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h3 id="注意点"><a href="#注意点" class="header-anchor">#</a> 注意点</h3> <p><strong>（1）浅拷贝</strong></p> <p><code>Object.assign</code>方法实行的是浅拷贝，而不是深拷贝。也就是说，如果源对象某个属性的值是对象，那么目标对象拷贝得到的是这个对象的引用。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> obj1 <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token punctuation">{</span><span class="token literal-property property">b</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">}</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> obj2 <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> obj1<span class="token punctuation">)</span><span class="token punctuation">;</span>

obj1<span class="token punctuation">.</span>a<span class="token punctuation">.</span>b <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span>
obj2<span class="token punctuation">.</span>a<span class="token punctuation">.</span>b <span class="token comment">// 2</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，源对象<code>obj1</code>的<code>a</code>属性的值是一个对象，<code>Object.assign</code>拷贝得到的是这个对象的引用。这个对象的任何变化，都会反映到目标对象上面。</p> <p><strong>（2）同名属性的替换</strong></p> <p>对于这种嵌套的对象，一旦遇到同名属性，<code>Object.assign</code>的处理方法是替换，而不是添加。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> target <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token punctuation">{</span> <span class="token literal-property property">b</span><span class="token operator">:</span> <span class="token string">'c'</span><span class="token punctuation">,</span> <span class="token literal-property property">d</span><span class="token operator">:</span> <span class="token string">'e'</span> <span class="token punctuation">}</span> <span class="token punctuation">}</span>
<span class="token keyword">const</span> source <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token punctuation">{</span> <span class="token literal-property property">b</span><span class="token operator">:</span> <span class="token string">'hello'</span> <span class="token punctuation">}</span> <span class="token punctuation">}</span>
Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> source<span class="token punctuation">)</span>
<span class="token comment">// { a: { b: 'hello' } }</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，<code>target</code>对象的<code>a</code>属性被<code>source</code>对象的<code>a</code>属性整个替换掉了，而不会得到<code>{ a: { b: 'hello', d: 'e' } }</code>的结果。这通常不是开发者想要的，需要特别小心。</p> <p>一些函数库提供<code>Object.assign</code>的定制版本（比如 Lodash 的<code>_.defaultsDeep</code>方法），可以得到深拷贝的合并。</p> <p><strong>（3）数组的处理</strong></p> <p><code>Object.assign</code>可以用来处理数组，但是会把数组视为对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
<span class="token comment">// [4, 5, 3]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，<code>Object.assign</code>把数组视为属性名为 0、1、2 的对象，因此源数组的 0 号属性<code>4</code>覆盖了目标数组的 0 号属性<code>1</code>。</p> <p><strong>（4）取值函数的处理</strong></p> <p><code>Object.assign</code>只能进行值的复制，如果要复制的值是一个取值函数，那么将求值后再复制。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> source <span class="token operator">=</span> <span class="token punctuation">{</span>
  <span class="token keyword">get</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token number">1</span> <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> target <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>

Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> source<span class="token punctuation">)</span>
<span class="token comment">// { foo: 1 }</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，<code>source</code>对象的<code>foo</code>属性是一个取值函数，<code>Object.assign</code>不会复制这个取值函数，只会拿到值以后，将这个值复制过去。</p> <h3 id="常见用途"><a href="#常见用途" class="header-anchor">#</a> 常见用途</h3> <p><code>Object.assign</code>方法有很多用处。</p> <p><strong>（1）为对象添加属性</strong></p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">Point</span> <span class="token punctuation">{</span>
  <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">x<span class="token punctuation">,</span> y</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>x<span class="token punctuation">,</span> y<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面方法通过<code>Object.assign</code>方法，将<code>x</code>属性和<code>y</code>属性添加到<code>Point</code>类的对象实例。</p> <p><strong>（2）为对象添加方法</strong></p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span><span class="token class-name">SomeClass</span><span class="token punctuation">.</span>prototype<span class="token punctuation">,</span> <span class="token punctuation">{</span>
  <span class="token function">someMethod</span><span class="token punctuation">(</span><span class="token parameter">arg1<span class="token punctuation">,</span> arg2</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    ···
  <span class="token punctuation">}</span><span class="token punctuation">,</span>
  <span class="token function">anotherMethod</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    ···
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// 等同于下面的写法</span>
<span class="token class-name">SomeClass</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function-variable function">someMethod</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">arg1<span class="token punctuation">,</span> arg2</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  ···
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token class-name">SomeClass</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function-variable function">anotherMethod</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  ···
<span class="token punctuation">}</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br></div></div><p>上面代码使用了对象属性的简洁表示法，直接将两个函数放在大括号中，再使用<code>assign</code>方法添加到<code>SomeClass.prototype</code>之中。</p> <p><strong>（3）克隆对象</strong></p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">clone</span><span class="token punctuation">(</span><span class="token parameter">origin</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> origin<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码将原始对象拷贝到一个空对象，就得到了原始对象的克隆。</p> <p>不过，采用这种方法克隆，只能克隆原始对象自身的值，不能克隆它继承的值。如果想要保持继承链，可以采用下面的代码。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">clone</span><span class="token punctuation">(</span><span class="token parameter">origin</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">let</span> originProto <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">getPrototypeOf</span><span class="token punctuation">(</span>origin<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">return</span> Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span>Object<span class="token punctuation">.</span><span class="token function">create</span><span class="token punctuation">(</span>originProto<span class="token punctuation">)</span><span class="token punctuation">,</span> origin<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p><strong>（4）合并多个对象</strong></p> <p>将多个对象合并到某个对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> <span class="token function-variable function">merge</span> <span class="token operator">=</span>
  <span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> <span class="token operator">...</span>sources</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> <span class="token operator">...</span>sources<span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>如果希望合并后返回一个新对象，可以改写上面函数，对一个空对象合并。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> <span class="token function-variable function">merge</span> <span class="token operator">=</span>
  <span class="token punctuation">(</span><span class="token parameter"><span class="token operator">...</span>sources</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token operator">...</span>sources<span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p><strong>（5）为属性指定默认值</strong></p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> <span class="token constant">DEFAULTS</span> <span class="token operator">=</span> <span class="token punctuation">{</span>
  <span class="token literal-property property">logLevel</span><span class="token operator">:</span> <span class="token number">0</span><span class="token punctuation">,</span>
  <span class="token literal-property property">outputFormat</span><span class="token operator">:</span> <span class="token string">'html'</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token keyword">function</span> <span class="token function">processContent</span><span class="token punctuation">(</span><span class="token parameter">options</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  options <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token constant">DEFAULTS</span><span class="token punctuation">,</span> options<span class="token punctuation">)</span><span class="token punctuation">;</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>options<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token comment">// ...</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码中，<code>DEFAULTS</code>对象是默认值，<code>options</code>对象是用户提供的参数。<code>Object.assign</code>方法将<code>DEFAULTS</code>和<code>options</code>合并成一个新对象，如果两者有同名属性，则<code>options</code>的属性值会覆盖<code>DEFAULTS</code>的属性值。</p> <p>注意，由于存在浅拷贝的问题，<code>DEFAULTS</code>对象和<code>options</code>对象的所有属性的值，最好都是简单类型，不要指向另一个对象。否则，<code>DEFAULTS</code>对象的该属性很可能不起作用。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> <span class="token constant">DEFAULTS</span> <span class="token operator">=</span> <span class="token punctuation">{</span>
  <span class="token literal-property property">url</span><span class="token operator">:</span> <span class="token punctuation">{</span>
    <span class="token literal-property property">host</span><span class="token operator">:</span> <span class="token string">'example.com'</span><span class="token punctuation">,</span>
    <span class="token literal-property property">port</span><span class="token operator">:</span> <span class="token number">7070</span>
  <span class="token punctuation">}</span><span class="token punctuation">,</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token function">processContent</span><span class="token punctuation">(</span><span class="token punctuation">{</span> <span class="token literal-property property">url</span><span class="token operator">:</span> <span class="token punctuation">{</span><span class="token literal-property property">port</span><span class="token operator">:</span> <span class="token number">8000</span><span class="token punctuation">}</span> <span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token comment">// {</span>
<span class="token comment">//   url: {port: 8000}</span>
<span class="token comment">// }</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面代码的原意是将<code>url.port</code>改成 8000，<code>url.host</code>不变。实际结果却是<code>options.url</code>覆盖掉<code>DEFAULTS.url</code>，所以<code>url.host</code>就不存在了。</p> <h2 id="object-getownpropertydescriptors"><a href="#object-getownpropertydescriptors" class="header-anchor">#</a> Object.getOwnPropertyDescriptors()</h2> <p>ES5 的<code>Object.getOwnPropertyDescriptor()</code>方法会返回某个对象属性的描述对象（descriptor）。ES2017 引入了<code>Object.getOwnPropertyDescriptors()</code>方法，返回指定对象所有自身属性（非继承属性）的描述对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
  <span class="token literal-property property">foo</span><span class="token operator">:</span> <span class="token number">123</span><span class="token punctuation">,</span>
  <span class="token keyword">get</span> <span class="token function">bar</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token string">'abc'</span> <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptors</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span>
<span class="token comment">// { foo:</span>
<span class="token comment">//    { value: 123,</span>
<span class="token comment">//      writable: true,</span>
<span class="token comment">//      enumerable: true,</span>
<span class="token comment">//      configurable: true },</span>
<span class="token comment">//   bar:</span>
<span class="token comment">//    { get: [Function: get bar],</span>
<span class="token comment">//      set: undefined,</span>
<span class="token comment">//      enumerable: true,</span>
<span class="token comment">//      configurable: true } }</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br></div></div><p>上面代码中，<code>Object.getOwnPropertyDescriptors()</code>方法返回一个对象，所有原对象的属性名都是该对象的属性名，对应的属性值就是该属性的描述对象。</p> <p>该方法的实现非常容易。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">getOwnPropertyDescriptors</span><span class="token punctuation">(</span><span class="token parameter">obj</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> result <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
  <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> key <span class="token keyword">of</span> Reflect<span class="token punctuation">.</span><span class="token function">ownKeys</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    result<span class="token punctuation">[</span>key<span class="token punctuation">]</span> <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptor</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> key<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
  <span class="token keyword">return</span> result<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>该方法的引入目的，主要是为了解决<code>Object.assign()</code>无法正确拷贝<code>get</code>属性和<code>set</code>属性的问题。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> source <span class="token operator">=</span> <span class="token punctuation">{</span>
  <span class="token keyword">set</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token keyword">const</span> target1 <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span>target1<span class="token punctuation">,</span> source<span class="token punctuation">)</span><span class="token punctuation">;</span>

Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptor</span><span class="token punctuation">(</span>target1<span class="token punctuation">,</span> <span class="token string">'foo'</span><span class="token punctuation">)</span>
<span class="token comment">// { value: undefined,</span>
<span class="token comment">//   writable: true,</span>
<span class="token comment">//   enumerable: true,</span>
<span class="token comment">//   configurable: true }</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br></div></div><p>上面代码中，<code>source</code>对象的<code>foo</code>属性的值是一个赋值函数，<code>Object.assign</code>方法将这个属性拷贝给<code>target1</code>对象，结果该属性的值变成了<code>undefined</code>。这是因为<code>Object.assign</code>方法总是拷贝一个属性的值，而不会拷贝它背后的赋值方法或取值方法。</p> <p>这时，<code>Object.getOwnPropertyDescriptors()</code>方法配合<code>Object.defineProperties()</code>方法，就可以实现正确拷贝。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> source <span class="token operator">=</span> <span class="token punctuation">{</span>
  <span class="token keyword">set</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token keyword">const</span> target2 <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
Object<span class="token punctuation">.</span><span class="token function">defineProperties</span><span class="token punctuation">(</span>target2<span class="token punctuation">,</span> Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptors</span><span class="token punctuation">(</span>source<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptor</span><span class="token punctuation">(</span>target2<span class="token punctuation">,</span> <span class="token string">'foo'</span><span class="token punctuation">)</span>
<span class="token comment">// { get: undefined,</span>
<span class="token comment">//   set: [Function: set foo],</span>
<span class="token comment">//   enumerable: true,</span>
<span class="token comment">//   configurable: true }</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div><p>上面代码中，两个对象合并的逻辑可以写成一个函数。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> <span class="token function-variable function">shallowMerge</span> <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> source</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> Object<span class="token punctuation">.</span><span class="token function">defineProperties</span><span class="token punctuation">(</span>
  target<span class="token punctuation">,</span>
  Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptors</span><span class="token punctuation">(</span>source<span class="token punctuation">)</span>
<span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p><code>Object.getOwnPropertyDescriptors()</code>方法的另一个用处，是配合<code>Object.create()</code>方法，将对象属性克隆到一个新对象。这属于浅拷贝。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> clone <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">create</span><span class="token punctuation">(</span>Object<span class="token punctuation">.</span><span class="token function">getPrototypeOf</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">,</span>
  Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptors</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// 或者</span>

<span class="token keyword">const</span> <span class="token function-variable function">shallowClone</span> <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token parameter">obj</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> Object<span class="token punctuation">.</span><span class="token function">create</span><span class="token punctuation">(</span>
  Object<span class="token punctuation">.</span><span class="token function">getPrototypeOf</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">,</span>
  Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptors</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span>
<span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>上面代码会克隆对象<code>obj</code>。</p> <p>另外，<code>Object.getOwnPropertyDescriptors()</code>方法可以实现一个对象继承另一个对象。以前，继承另一个对象，常常写成下面这样。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
  <span class="token literal-property property">__proto__</span><span class="token operator">:</span> prot<span class="token punctuation">,</span>
  <span class="token literal-property property">foo</span><span class="token operator">:</span> <span class="token number">123</span><span class="token punctuation">,</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>ES6 规定<code>__proto__</code>只有浏览器要部署，其他环境不用部署。如果去除<code>__proto__</code>，上面代码就要改成下面这样。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> obj <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">create</span><span class="token punctuation">(</span>prot<span class="token punctuation">)</span><span class="token punctuation">;</span>
obj<span class="token punctuation">.</span>foo <span class="token operator">=</span> <span class="token number">123</span><span class="token punctuation">;</span>

<span class="token comment">// 或者</span>

<span class="token keyword">const</span> obj <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span>
  Object<span class="token punctuation">.</span><span class="token function">create</span><span class="token punctuation">(</span>prot<span class="token punctuation">)</span><span class="token punctuation">,</span>
  <span class="token punctuation">{</span>
    <span class="token literal-property property">foo</span><span class="token operator">:</span> <span class="token number">123</span><span class="token punctuation">,</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>有了<code>Object.getOwnPropertyDescriptors()</code>，我们就有了另一种写法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> obj <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">create</span><span class="token punctuation">(</span>
  prot<span class="token punctuation">,</span>
  Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptors</span><span class="token punctuation">(</span><span class="token punctuation">{</span>
    <span class="token literal-property property">foo</span><span class="token operator">:</span> <span class="token number">123</span><span class="token punctuation">,</span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p><code>Object.getOwnPropertyDescriptors()</code>也可以用来实现 Mixin（混入）模式。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> <span class="token function-variable function">mix</span> <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token parameter">object</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">(</span><span class="token punctuation">{</span>
  <span class="token function-variable function">with</span><span class="token operator">:</span> <span class="token punctuation">(</span><span class="token parameter"><span class="token operator">...</span>mixins</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> mixins<span class="token punctuation">.</span><span class="token function">reduce</span><span class="token punctuation">(</span>
    <span class="token punctuation">(</span><span class="token parameter">c<span class="token punctuation">,</span> mixin</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> Object<span class="token punctuation">.</span><span class="token function">create</span><span class="token punctuation">(</span>
      c<span class="token punctuation">,</span> Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptors</span><span class="token punctuation">(</span>mixin<span class="token punctuation">)</span>
    <span class="token punctuation">)</span><span class="token punctuation">,</span> object<span class="token punctuation">)</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// multiple mixins example</span>
<span class="token keyword">let</span> a <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token string">'a'</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> b <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">b</span><span class="token operator">:</span> <span class="token string">'b'</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> c <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">c</span><span class="token operator">:</span> <span class="token string">'c'</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> d <span class="token operator">=</span> <span class="token function">mix</span><span class="token punctuation">(</span>c<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">with</span><span class="token punctuation">(</span>a<span class="token punctuation">,</span> b<span class="token punctuation">)</span><span class="token punctuation">;</span>

d<span class="token punctuation">.</span>c <span class="token comment">// &quot;c&quot;</span>
d<span class="token punctuation">.</span>b <span class="token comment">// &quot;b&quot;</span>
d<span class="token punctuation">.</span>a <span class="token comment">// &quot;a&quot;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br></div></div><p>上面代码返回一个新的对象<code>d</code>，代表了对象<code>a</code>和<code>b</code>被混入了对象<code>c</code>的操作。</p> <p>出于完整性的考虑，<code>Object.getOwnPropertyDescriptors()</code>进入标准以后，以后还会新增<code>Reflect.getOwnPropertyDescriptors()</code>方法。</p> <h2 id="proto-属性-object-setprototypeof-object-getprototypeof"><a href="#proto-属性-object-setprototypeof-object-getprototypeof" class="header-anchor">#</a> <code>__proto__</code>属性，Object.setPrototypeOf()，Object.getPrototypeOf()</h2> <p>JavaScript 语言的对象继承是通过原型链实现的。ES6 提供了更多原型对象的操作方法。</p> <h3 id="proto-属性"><a href="#proto-属性" class="header-anchor">#</a> <code>__proto__</code>属性</h3> <p><code>__proto__</code>属性（前后各两个下划线），用来读取或设置当前对象的<code>prototype</code>对象。目前，所有浏览器（包括 IE11）都部署了这个属性。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// es5 的写法</span>
<span class="token keyword">const</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
  <span class="token function-variable function">method</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token operator">...</span> <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
obj<span class="token punctuation">.</span>__proto__ <span class="token operator">=</span> someOtherObj<span class="token punctuation">;</span>

<span class="token comment">// es6 的写法</span>
<span class="token keyword">var</span> obj <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">create</span><span class="token punctuation">(</span>someOtherObj<span class="token punctuation">)</span><span class="token punctuation">;</span>
obj<span class="token punctuation">.</span><span class="token function-variable function">method</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token operator">...</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>该属性没有写入 ES6 的正文，而是写入了附录，原因是<code>__proto__</code>前后的双下划线，说明它本质上是一个内部属性，而不是一个正式的对外的 API，只是由于浏览器广泛支持，才被加入了 ES6。标准明确规定，只有浏览器必须部署这个属性，其他运行环境不一定需要部署，而且新的代码最好认为这个属性是不存在的。因此，无论从语义的角度，还是从兼容性的角度，都不要使用这个属性，而是使用下面的<code>Object.setPrototypeOf()</code>（写操作）、<code>Object.getPrototypeOf()</code>（读操作）、<code>Object.create()</code>（生成操作）代替。</p> <p>实现上，<code>__proto__</code>调用的是<code>Object.prototype.__proto__</code>，具体实现如下。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span><span class="token class-name">Object</span><span class="token punctuation">.</span>prototype<span class="token punctuation">,</span> <span class="token string">'__proto__'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
  <span class="token function">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">let</span> _thisObj <span class="token operator">=</span> <span class="token function">Object</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> Object<span class="token punctuation">.</span><span class="token function">getPrototypeOf</span><span class="token punctuation">(</span>_thisObj<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">,</span>
  <span class="token function">set</span><span class="token punctuation">(</span>proto<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">this</span> <span class="token operator">===</span> <span class="token keyword">undefined</span> <span class="token operator">||</span> <span class="token keyword">this</span> <span class="token operator">===</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">TypeError</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span><span class="token function">isObject</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">return</span> <span class="token keyword">undefined</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span><span class="token function">isObject</span><span class="token punctuation">(</span>proto<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">return</span> <span class="token keyword">undefined</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">let</span> status <span class="token operator">=</span> Reflect<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">,</span> proto<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>status<span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">TypeError</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span><span class="token punctuation">,</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">function</span> <span class="token function">isObject</span><span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> <span class="token function">Object</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span> <span class="token operator">===</span> value<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br></div></div><p>如果一个对象本身部署了<code>__proto__</code>属性，该属性的值就是对象的原型。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function">getPrototypeOf</span><span class="token punctuation">(</span><span class="token punctuation">{</span> <span class="token literal-property property">__proto__</span><span class="token operator">:</span> <span class="token keyword">null</span> <span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token comment">// null</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h3 id="object-setprototypeof"><a href="#object-setprototypeof" class="header-anchor">#</a> Object.setPrototypeOf()</h3> <p><code>Object.setPrototypeOf</code>方法的作用与<code>__proto__</code>相同，用来设置一个对象的<code>prototype</code>对象，返回参数对象本身。它是 ES6 正式推荐的设置原型对象的方法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 格式</span>
Object<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span>object<span class="token punctuation">,</span> prototype<span class="token punctuation">)</span>

<span class="token comment">// 用法</span>
<span class="token keyword">const</span> o <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>该方法等同于下面的函数。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">setPrototypeOf</span><span class="token punctuation">(</span><span class="token parameter">obj<span class="token punctuation">,</span> proto</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  obj<span class="token punctuation">.</span>__proto__ <span class="token operator">=</span> proto<span class="token punctuation">;</span>
  <span class="token keyword">return</span> obj<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>下面是一个例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> proto <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">x</span><span class="token operator">:</span> <span class="token number">10</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
Object<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> proto<span class="token punctuation">)</span><span class="token punctuation">;</span>

proto<span class="token punctuation">.</span>y <span class="token operator">=</span> <span class="token number">20</span><span class="token punctuation">;</span>
proto<span class="token punctuation">.</span>z <span class="token operator">=</span> <span class="token number">40</span><span class="token punctuation">;</span>

obj<span class="token punctuation">.</span>x <span class="token comment">// 10</span>
obj<span class="token punctuation">.</span>y <span class="token comment">// 20</span>
obj<span class="token punctuation">.</span>z <span class="token comment">// 40</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码将<code>proto</code>对象设为<code>obj</code>对象的原型，所以从<code>obj</code>对象可以读取<code>proto</code>对象的属性。</p> <p>如果第一个参数不是对象，会自动转为对象。但是由于返回的还是第一个参数，所以这个操作不会产生任何效果。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token operator">===</span> <span class="token number">1</span> <span class="token comment">// true</span>
Object<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span><span class="token string">'foo'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token operator">===</span> <span class="token string">'foo'</span> <span class="token comment">// true</span>
Object<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token operator">===</span> <span class="token boolean">true</span> <span class="token comment">// true</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>由于<code>undefined</code>和<code>null</code>无法转为对象，所以如果第一个参数是<code>undefined</code>或<code>null</code>，就会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span><span class="token keyword">undefined</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token comment">// TypeError: Object.setPrototypeOf called on null or undefined</span>

Object<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token comment">// TypeError: Object.setPrototypeOf called on null or undefined</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><h3 id="object-getprototypeof"><a href="#object-getprototypeof" class="header-anchor">#</a> Object.getPrototypeOf()</h3> <p>该方法与<code>Object.setPrototypeOf</code>方法配套，用于读取一个对象的原型对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function">getPrototypeOf</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>下面是一个例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">Rectangle</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token comment">// ...</span>
<span class="token punctuation">}</span>

<span class="token keyword">const</span> rec <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Rectangle</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

Object<span class="token punctuation">.</span><span class="token function">getPrototypeOf</span><span class="token punctuation">(</span>rec<span class="token punctuation">)</span> <span class="token operator">===</span> <span class="token class-name">Rectangle</span><span class="token punctuation">.</span>prototype
<span class="token comment">// true</span>

Object<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span>rec<span class="token punctuation">,</span> <span class="token class-name">Object</span><span class="token punctuation">.</span>prototype<span class="token punctuation">)</span><span class="token punctuation">;</span>
Object<span class="token punctuation">.</span><span class="token function">getPrototypeOf</span><span class="token punctuation">(</span>rec<span class="token punctuation">)</span> <span class="token operator">===</span> <span class="token class-name">Rectangle</span><span class="token punctuation">.</span>prototype
<span class="token comment">// false</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p>如果参数不是对象，会被自动转为对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 等同于 Object.getPrototypeOf(Number(1))</span>
Object<span class="token punctuation">.</span><span class="token function">getPrototypeOf</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span>
<span class="token comment">// Number {[[PrimitiveValue]]: 0}</span>

<span class="token comment">// 等同于 Object.getPrototypeOf(String('foo'))</span>
Object<span class="token punctuation">.</span><span class="token function">getPrototypeOf</span><span class="token punctuation">(</span><span class="token string">'foo'</span><span class="token punctuation">)</span>
<span class="token comment">// String {length: 0, [[PrimitiveValue]]: &quot;&quot;}</span>

<span class="token comment">// 等同于 Object.getPrototypeOf(Boolean(true))</span>
Object<span class="token punctuation">.</span><span class="token function">getPrototypeOf</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span>
<span class="token comment">// Boolean {[[PrimitiveValue]]: false}</span>

Object<span class="token punctuation">.</span><span class="token function">getPrototypeOf</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token operator">===</span> <span class="token class-name">Number</span><span class="token punctuation">.</span>prototype <span class="token comment">// true</span>
Object<span class="token punctuation">.</span><span class="token function">getPrototypeOf</span><span class="token punctuation">(</span><span class="token string">'foo'</span><span class="token punctuation">)</span> <span class="token operator">===</span> <span class="token class-name">String</span><span class="token punctuation">.</span>prototype <span class="token comment">// true</span>
Object<span class="token punctuation">.</span><span class="token function">getPrototypeOf</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token operator">===</span> <span class="token class-name">Boolean</span><span class="token punctuation">.</span>prototype <span class="token comment">// true</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br></div></div><p>如果参数是<code>undefined</code>或<code>null</code>，它们无法转为对象，所以会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function">getPrototypeOf</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">)</span>
<span class="token comment">// TypeError: Cannot convert undefined or null to object</span>

Object<span class="token punctuation">.</span><span class="token function">getPrototypeOf</span><span class="token punctuation">(</span><span class="token keyword">undefined</span><span class="token punctuation">)</span>
<span class="token comment">// TypeError: Cannot convert undefined or null to object</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><h2 id="object-keys-object-values-object-entries"><a href="#object-keys-object-values-object-entries" class="header-anchor">#</a> Object.keys()，Object.values()，Object.entries()</h2> <h3 id="object-keys"><a href="#object-keys" class="header-anchor">#</a> Object.keys()</h3> <p>ES5 引入了<code>Object.keys</code>方法，返回一个数组，成员是参数对象自身的（不含继承的）所有可遍历（enumerable）属性的键名。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">foo</span><span class="token operator">:</span> <span class="token string">'bar'</span><span class="token punctuation">,</span> <span class="token literal-property property">baz</span><span class="token operator">:</span> <span class="token number">42</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
Object<span class="token punctuation">.</span><span class="token function">keys</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span>
<span class="token comment">// [&quot;foo&quot;, &quot;baz&quot;]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>ES2017 <a href="https://github.com/tc39/proposal-object-values-entries" target="_blank" rel="noopener noreferrer">引入<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>了跟<code>Object.keys</code>配套的<code>Object.values</code>和<code>Object.entries</code>，作为遍历一个对象的补充手段，供<code>for...of</code>循环使用。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> <span class="token punctuation">{</span>keys<span class="token punctuation">,</span> values<span class="token punctuation">,</span> entries<span class="token punctuation">}</span> <span class="token operator">=</span> Object<span class="token punctuation">;</span>
<span class="token keyword">let</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token literal-property property">b</span><span class="token operator">:</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token literal-property property">c</span><span class="token operator">:</span> <span class="token number">3</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> key <span class="token keyword">of</span> <span class="token function">keys</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>key<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 'a', 'b', 'c'</span>
<span class="token punctuation">}</span>

<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> value <span class="token keyword">of</span> <span class="token function">values</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 1, 2, 3</span>
<span class="token punctuation">}</span>

<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> <span class="token punctuation">[</span>key<span class="token punctuation">,</span> value<span class="token punctuation">]</span> <span class="token keyword">of</span> <span class="token function">entries</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token punctuation">[</span>key<span class="token punctuation">,</span> value<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// ['a', 1], ['b', 2], ['c', 3]</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br></div></div><h3 id="object-values"><a href="#object-values" class="header-anchor">#</a> Object.values()</h3> <p><code>Object.values</code>方法返回一个数组，成员是参数对象自身的（不含继承的）所有可遍历（enumerable）属性的键值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">foo</span><span class="token operator">:</span> <span class="token string">'bar'</span><span class="token punctuation">,</span> <span class="token literal-property property">baz</span><span class="token operator">:</span> <span class="token number">42</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
Object<span class="token punctuation">.</span><span class="token function">values</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span>
<span class="token comment">// [&quot;bar&quot;, 42]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>返回数组的成员顺序，与本章的《属性的遍历》部分介绍的排列规则一致。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token number">100</span><span class="token operator">:</span> <span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token operator">:</span> <span class="token string">'b'</span><span class="token punctuation">,</span> <span class="token number">7</span><span class="token operator">:</span> <span class="token string">'c'</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
Object<span class="token punctuation">.</span><span class="token function">values</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span>
<span class="token comment">// [&quot;b&quot;, &quot;c&quot;, &quot;a&quot;]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码中，属性名为数值的属性，是按照数值大小，从小到大遍历的，因此返回的顺序是<code>b</code>、<code>c</code>、<code>a</code>。</p> <p><code>Object.values</code>只返回对象自身的可遍历属性。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> obj <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">create</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token literal-property property">p</span><span class="token operator">:</span> <span class="token punctuation">{</span><span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token number">42</span><span class="token punctuation">}</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
Object<span class="token punctuation">.</span><span class="token function">values</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span> <span class="token comment">// []</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，<code>Object.create</code>方法的第二个参数添加的对象属性（属性<code>p</code>），如果不显式声明，默认是不可遍历的，因为<code>p</code>的属性描述对象的<code>enumerable</code>默认是<code>false</code>，<code>Object.values</code>不会返回这个属性。只要把<code>enumerable</code>改成<code>true</code>，<code>Object.values</code>就会返回属性<code>p</code>的值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> obj <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">create</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token literal-property property">p</span><span class="token operator">:</span>
  <span class="token punctuation">{</span>
    value<span class="token operator">:</span> <span class="token number">42</span><span class="token punctuation">,</span>
    enumerable<span class="token operator">:</span> <span class="token boolean">true</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
Object<span class="token punctuation">.</span><span class="token function">values</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span> <span class="token comment">// [42]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p><code>Object.values</code>会过滤属性名为 Symbol 值的属性。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function">values</span><span class="token punctuation">(</span><span class="token punctuation">{</span> <span class="token punctuation">[</span><span class="token function">Symbol</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">]</span><span class="token operator">:</span> <span class="token number">123</span><span class="token punctuation">,</span> <span class="token literal-property property">foo</span><span class="token operator">:</span> <span class="token string">'abc'</span> <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// ['abc']</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>如果<code>Object.values</code>方法的参数是一个字符串，会返回各个字符组成的一个数组。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function">values</span><span class="token punctuation">(</span><span class="token string">'foo'</span><span class="token punctuation">)</span>
<span class="token comment">// ['f', 'o', 'o']</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，字符串会先转成一个类似数组的对象。字符串的每个字符，就是该对象的一个属性。因此，<code>Object.values</code>返回每个属性的键值，就是各个字符组成的一个数组。</p> <p>如果参数不是对象，<code>Object.values</code>会先将其转为对象。由于数值和布尔值的包装对象，都不会为实例添加非继承的属性。所以，<code>Object.values</code>会返回空数组。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function">values</span><span class="token punctuation">(</span><span class="token number">42</span><span class="token punctuation">)</span> <span class="token comment">// []</span>
Object<span class="token punctuation">.</span><span class="token function">values</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token comment">// []</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h3 id="object-entries"><a href="#object-entries" class="header-anchor">#</a> Object.entries()</h3> <p><code>Object.entries()</code>方法返回一个数组，成员是参数对象自身的（不含继承的）所有可遍历（enumerable）属性的键值对数组。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">foo</span><span class="token operator">:</span> <span class="token string">'bar'</span><span class="token punctuation">,</span> <span class="token literal-property property">baz</span><span class="token operator">:</span> <span class="token number">42</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
Object<span class="token punctuation">.</span><span class="token function">entries</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span>
<span class="token comment">// [ [&quot;foo&quot;, &quot;bar&quot;], [&quot;baz&quot;, 42] ]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>除了返回值不一样，该方法的行为与<code>Object.values</code>基本一致。</p> <p>如果原对象的属性名是一个 Symbol 值，该属性会被忽略。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function">entries</span><span class="token punctuation">(</span><span class="token punctuation">{</span> <span class="token punctuation">[</span><span class="token function">Symbol</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">]</span><span class="token operator">:</span> <span class="token number">123</span><span class="token punctuation">,</span> <span class="token literal-property property">foo</span><span class="token operator">:</span> <span class="token string">'abc'</span> <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// [ [ 'foo', 'abc' ] ]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，原对象有两个属性，<code>Object.entries</code>只输出属性名非 Symbol 值的属性。将来可能会有<code>Reflect.ownEntries()</code>方法，返回对象自身的所有属性。</p> <p><code>Object.entries</code>的基本用途是遍历对象的属性。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">one</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token literal-property property">two</span><span class="token operator">:</span> <span class="token number">2</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> <span class="token punctuation">[</span>k<span class="token punctuation">,</span> v<span class="token punctuation">]</span> <span class="token keyword">of</span> Object<span class="token punctuation">.</span><span class="token function">entries</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>
    <span class="token template-string"><span class="token template-punctuation string">`</span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span><span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span>k<span class="token punctuation">)</span><span class="token interpolation-punctuation punctuation">}</span></span><span class="token string">: </span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span><span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span>v<span class="token punctuation">)</span><span class="token interpolation-punctuation punctuation">}</span></span><span class="token template-punctuation string">`</span></span>
  <span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// &quot;one&quot;: 1</span>
<span class="token comment">// &quot;two&quot;: 2</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p><code>Object.entries</code>方法的另一个用处是，将对象转为真正的<code>Map</code>结构。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">foo</span><span class="token operator">:</span> <span class="token string">'bar'</span><span class="token punctuation">,</span> <span class="token literal-property property">baz</span><span class="token operator">:</span> <span class="token number">42</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> map <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Map</span><span class="token punctuation">(</span>Object<span class="token punctuation">.</span><span class="token function">entries</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
map <span class="token comment">// Map { foo: &quot;bar&quot;, baz: 42 }</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>自己实现<code>Object.entries</code>方法，非常简单。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// Generator函数的版本</span>
<span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">entries</span><span class="token punctuation">(</span><span class="token parameter">obj</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> key <span class="token keyword">of</span> Object<span class="token punctuation">.</span><span class="token function">keys</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">yield</span> <span class="token punctuation">[</span>key<span class="token punctuation">,</span> obj<span class="token punctuation">[</span>key<span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

<span class="token comment">// 非Generator函数的版本</span>
<span class="token keyword">function</span> <span class="token function">entries</span><span class="token punctuation">(</span><span class="token parameter">obj</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">let</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
  <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> key <span class="token keyword">of</span> Object<span class="token punctuation">.</span><span class="token function">keys</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    arr<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span><span class="token punctuation">[</span>key<span class="token punctuation">,</span> obj<span class="token punctuation">[</span>key<span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
  <span class="token keyword">return</span> arr<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br></div></div><h2 id="object-fromentries"><a href="#object-fromentries" class="header-anchor">#</a> Object.fromEntries()</h2> <p><code>Object.fromEntries()</code>方法是<code>Object.entries()</code>的逆操作，用于将一个键值对数组转为对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function">fromEntries</span><span class="token punctuation">(</span><span class="token punctuation">[</span>
  <span class="token punctuation">[</span><span class="token string">'foo'</span><span class="token punctuation">,</span> <span class="token string">'bar'</span><span class="token punctuation">]</span><span class="token punctuation">,</span>
  <span class="token punctuation">[</span><span class="token string">'baz'</span><span class="token punctuation">,</span> <span class="token number">42</span><span class="token punctuation">]</span>
<span class="token punctuation">]</span><span class="token punctuation">)</span>
<span class="token comment">// { foo: &quot;bar&quot;, baz: 42 }</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>该方法的主要目的，是将键值对的数据结构还原为对象，因此特别适合将 Map 结构转为对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 例一</span>
<span class="token keyword">const</span> entries <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Map</span><span class="token punctuation">(</span><span class="token punctuation">[</span>
  <span class="token punctuation">[</span><span class="token string">'foo'</span><span class="token punctuation">,</span> <span class="token string">'bar'</span><span class="token punctuation">]</span><span class="token punctuation">,</span>
  <span class="token punctuation">[</span><span class="token string">'baz'</span><span class="token punctuation">,</span> <span class="token number">42</span><span class="token punctuation">]</span>
<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

Object<span class="token punctuation">.</span><span class="token function">fromEntries</span><span class="token punctuation">(</span>entries<span class="token punctuation">)</span>
<span class="token comment">// { foo: &quot;bar&quot;, baz: 42 }</span>

<span class="token comment">// 例二</span>
<span class="token keyword">const</span> map <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Map</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">set</span><span class="token punctuation">(</span><span class="token string">'foo'</span><span class="token punctuation">,</span> <span class="token boolean">true</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">set</span><span class="token punctuation">(</span><span class="token string">'bar'</span><span class="token punctuation">,</span> <span class="token boolean">false</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
Object<span class="token punctuation">.</span><span class="token function">fromEntries</span><span class="token punctuation">(</span>map<span class="token punctuation">)</span>
<span class="token comment">// { foo: true, bar: false }</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div><p>该方法的一个用处是配合<code>URLSearchParams</code>对象，将查询字符串转为对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function">fromEntries</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">URLSearchParams</span><span class="token punctuation">(</span><span class="token string">'foo=bar&amp;baz=qux'</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
<span class="token comment">// { foo: &quot;bar&quot;, baz: &quot;qux&quot; }</span>
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